The general aim of my work is to try to learn what the cerebellum contributes to movement and posture by recording with extracellular microelectrodes the discharge of single neurons in the brains of awake rhesus monkeys as they maintain postures and perform movements. At the present time, three projects are underway to attempt to answer the following questions: 1) What is the timing of the dentate nucleus? In man and higher monkeys, the largest of the cerebellar nuclei is the dentate. Given that it is reciprocally connected to motor cortex (both sending to it and receiving from it) the question is "Does it (in the initiation of movement) precede and help cause the first changes in motor cortex or does it follow and modify changes that have already occurred? Monkeys have been trained to move quickly while single unit discharges are recorded one at a time in dentate and motor cortex; so far, on the average, changes in dentate appear to precede the changes in motor cortex. Experiments are underway that involve ablation of dentate to show a causal as well as temporal relationship of dentate and motor cortex. 2) What is the coding of dentate output? Using a similar methodology, experiments have begun that examine dentate activity in specific relation to forces exerted by the arm and positions attained by the arm to see if it preferentially relates to one of the two. 3) Does the cerebellum participate in the learning of movements as well as their performance? The two spikes (simple and complex) of the Purkinje cell are recorded over the period of time that it takes a monkey to learn to perform several types of movement. The aim is to see whether there are any of the long-term changes in the occurrence of simple and complex spikes that are predicted by the Marr-Ito hypothesis of cerebellar learning.